It is well known that radio waves, propagating from a transmitter to a receiver, can follow a plurality of different paths, and that the relative phase of the different waves arriving at the receiving antenna can be such as to destructively interfere, causing what is commonly referred to as a fade. In order to protect against this condition, the so-called "space diversity" system has been developed using two, spaced antennas to feed a diversity receiver. The theory underlying the use of two spaced-apart antennas is that there is less likelihood that a fade will occur at both antennas at the same time. In the simplest system, means are provided to disconnect the receiver from one antenna as soon as the received signal level falls below a predetermined threshold and to connect the receiver to the second antenna. In this so-called "blind switching," it is assumed that the signal received by the second antenna is stronger than that received by the first antenna. In a more sophisticated system, the signals from the two antennas are combined instead of switching between the two. This eliminates amplitude and phase jumps associated with the switching operation, and has the added advantage of delivering a larger amplitude signal to the receiver.
In analogue radio systems, it is the practice to adjust the phases of the two received signals so as to maximize the total power in the combined signal. For example, in U.S. Pat. No. 2,786,133 the relative phase of the local oscillator signals coupled to a pair of frequency converters is adjusted such that the resulting IF signals produced combine to maximize the total signal.
Studies have shown, however, that the situation is more complex in a digital radio system. In particular, it has been found that the bit error rate (BER) in a digital system is more sensitive to the degree of inband amplitude dispersion and less sensitive to the magnitude of a flat fade. Accordingly, in the copending application by Barnett et al, Ser. No. 57,748, filed July 16, 1979, and now U.S. Pat. No. 4,261,056 and assigned to applicant's assignee, there is disclosed an equalizing combiner which trades output power in favor of reduced amplitude dispersion by measuring the power in selected portions of the output signal and varying the circuit parameters in a manner to maintain any preselected relation among the measured signal components. There are, however, situations wherein the phase difference between the two received signals is constant across the band of interest. In that case, amplitude dispersion cannot be corrected simply by varying the phase of one signal relative to the other.